Digital weighing system

ABSTRACT

A system is disclosed for providing digital indications of the gross or net weights of a load (or accumulated loads), as sensed by electrical load cells. The load cells are energized from a source of reference potential to provide an output signal that is converted to a digital form by a ratioed ramp voltmeter operating in cooperation with a digital counter. The ramp voltmeter is driven by reference potential to provide improved accuracy. An active multipole filter is provided for filtering the signal from the load cell structure to eliminate spurious components thereof created by vibration and the like. The system also incorporates, analogue and digital scaling, flexibility in the use of tare, and a test conversion facility.

United States Patent Dana Laboratories, Inc.

Appl. No. Filed Patented Assignee DIGITAL WEIGHING SYSTEM 9 Claims, 3Drawing Figs.

US. Cl

177/164, 177/210, 177/211 1 11. c1. 001 3/14, GOlg l9/52,G01g 23/16References Cited UNITED STATES PATENTS 'RE.24,'969 4/1961 GoldingPrimary Examiner-Robert S. Ward, .1 r. Attorney-Nilsson, Robbins, Wills& Berliner ABSTRACT: A system is disclosed for providing digitalindications of the gross or net weights of a load (or accumulatedloads), as sensed by electrical load cells. The load cells are energizedfrom a source of reference potential to provide an output signal that isconverted to a digital form by a ratioed ramp voltmeter operating incooperation with a digital counter. The ramp voltmeter is driven byreference potential to provide improved accuracy. An active multipolefilter is provided for filtering the signal from the load cell structureto eliminate spurious components thereof created by vibration and thelike. The system also incorporates, analogue and digital scaling,flexibility in the use of tare, and a test conversion facility.

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BACKGROUND AND SUMMARY OF THE INVENTION The development ofvariable-resistance load cells and similar units has resulted in avariety of electrical systems for indicating weights. Rather typically,these systems include a rigid platform supported by at least oneelectrical load cell,

which is energized to develop an electrical signal that is indicative ofthe weight of the applied load. One form of such a system is shown anddescribed in applicants copending US. Pat. application Ser. No. 588,516entitled -Force Transducer Output Measuring System Employing RatioTechnique.

Although various forms of prior systems have functioned effectively andeconomically, a corisiderable'need remains for a reliable system whichis flexible in operation and is capable of accurately indicating gross,net, and-totalized (accumulated) weights in digital form. Such a systemwould desirably incorporate checking means (to verify that none of thecomponents 'have changed to significantly alter the accuracy ofcalibration) analogue and digital scaling capability, and flexibility inprogramming the tare. In general, the present invention fulfills theneeds utilizing a ratioed-ramp analogue-to-digital converter having afilter input. The system also includes sealing means and means forsimply and easily checking calibration. Tare weight signals, in digitalform are provided for flexible use in combination with gross-indicatingsignals.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, which constitute apart of this specification, an exemplary embodiment demonstratingvarious objectives andfeatures hereof is set forth, specifically:

FIG. 1 is a perspective and schematic view of a weight-indicating systemconstructed in accordance with the present invention; r

FIG. 2 is a schematic block and circuit diagram illustrating anelectrical system embodied in the structure of FIG. 1; and

FIG. 3 is a block diagram of a portion of the system of FIG. 2.

DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENT Referring initially to FIG.1, the load is shown borne in a container 11 whichis supported on aplatform 12 that is in turn carried by at least one load cell 14 asindicated by a dashed line 15. Of course, a single load cell may beused, or several may be variously disposed to support the platform 12,as well known in the art. As shown, the load cell 14 is supported on abase 16 which defines a space within which the platform 12 is received.

The load cell 14 is connected through an electrical cable 18 (indicatedby a solid line) to a remote 'unit 20 incorporating a frontinstrumentation panel 22. The 'panel includes a digital display window24 along with several other controls and indicators. Specifically, thepanel incorporates an foff/on" switch 26, a tare weight dial 27, anoperate/test switch 28, an overload lamp 29, a gross/net" switch 30, anaccumulate/single read" switch 32, an analogue scale dial 34, a digitalscale dial 36 and a pushbutton 38 which commands an actual' operationsequence to cause the system to indicate a weight. For example, thegross or net weight of the total load may be indicated by thedigital-display window 24.

After being turned on" the system is operated by pressing the weighpushbutton 38. The load 10 and the container 11 along with the platform12 then determinethe electrical signal provided from the load cell 14.The portion of the signal contributed by the platform 12 is balanced outand the resulting signal is processed to establish a digital display inthe window 24. Depending on the manner in which the system is programmedto operate, either tare or gross weights may be displayed as may variouscombinations thereof. For example, a tare weight may be set on the dial27 and the system can be operated to subtract such value from adigitally represented gross value so that a net weight is displayed inthe window 24.

known structures as a memory, a card reader or various other means toprovide digital tare-weight signals selectively in accordance with aspecific container 11.

Other operating programs for the system include the accumulation anddisplay of a plurality of individual weights with or without tareconsiderations. For example, a tare might be established as a goal fromwhich individual weights are subtracted until the display in the window24 is zeroed. Alternatively, a series of weights might be accumulatedand presented as a current sum in the window 24.

In programming the system, the value of the tare is established by thedial 27. Whether or not the tare value is employed is determined by theswitch 30 and whether the system accumulates or makes individualreadings is controlled by the switch 32. For example, placing the switch30 in the gross" position causes the system to neglect the tare. In thenet position,'the switch 30 causes the system to compute the differencebetween gross and tare 'to present the net. As indicated above,valuescan' be accumulated for display in the window 24. Calibration ofthe system is verified by placing the switch 28 in a test" position thenpressing the pushbutton 38 to produce indications to confirm a prior.calibration, or alternatively indicate thata deviation has occurred.

Turning now to the detailed system, reference will be made initially toFIG. 2 in which the digital display is provided by a digital outputapparatus 40 (incorporating a display not shown). The system isenergized by a power supply 42 (upper left) which incorporates theon/off" switch 26 and which may supply electrical power to all units inthe system in addition to those specifically indicated. However, in theinterest of avoiding unnecessary complexity in they drawing, many of thepower supply connections are omitted in accordance with conventionalpractice.

The power supply 42 is shown to be directly connected to a load-cellbridge circuit 43 including strain-gauge resistance elements 44, 46, 48and 50 connected in a bridge configuration as well known in theelectrical art. The conductors 52 and 54 from the power supply referencepotential at junction points 56 and 58 (of the bridge) which are locatedbetween the resistance elements 44 and 46 and the resistance elements 48and 50, respectively. The junction points 56 and 58 are also connectedto a ramp generator 60 and across voltage dividers 61 and 62. Thus, thebridge circuit 43, the voltage dividers 61 and 62, and the rampgenerator 60 are all directly energized by reference potential from thepower supply 42. A Furthermore, a threshold circuit 45 also connected tothe power supply 42, for ratio operation in sensing overloads.

The bridge circuit 43 is incorporated in the load cell 14 (FIG. 1) andprovides a signal across conductors 66 and 68 (FIG. 2) that isindicative of the load 10 and the container 11. In this regard, it is tobe noted that the signal component contributed by the platform 12 isbalanced out by the voltage divider 61. Specifically, the bridge circuit43 is connected to the 1 tap or movable contactof a potentiometer 63which is serially connected with resistors 64 and 65 to provide anadjustment that eliminates the signal component resulting from platformweight. Consequently, the conductors 66 and 68 (from the bridge circuit43) provide a voltage differential to'stationary contacts 70 and 72,that is representative of the load 10 and the container 11.

The contacts 70 and 72 are part of a double-pole doublethrow switch 77,including movable contacts 74 and 76, respectively. Another pair ofstationary contacts 78 and 80 (associated individually with the movablecontacts 74 and 76) are connected respectively to a tap 82 of apotentiometer 81 (in the voltage divider 62) and to a junction betweenresistors 83 and 85 (also in the divider 62). The movable contacts 74and 76 of the switches are connected through an adjustable amplifier 86and a filter 88, then through conductors 92 to a differential comparator90. The threshold circuit 45 is also connected across the conductors 92to establish a ratio threshold, above which the lamp 29 (FIGS. 1 and 2)is energized to indicate an overload.

The amplifier 86 (FIG. 2) functions to process the received I signal aswell known in the prior art and may be adjusted to accomplish afull-scale analogue signal, for application to the input of the filter88 in which alternating components are removed from the informationsignal .,When the load 10 and 1 comparator 90 operates with the rampgenerator 60 and an associated structure, to develop digital signalsrepresentative of the gross weight in gross register97. The tare weight(provided by a register 98) may be subtracted from the gross, asdescribed below, to accomplish adigital presentationof the net weight bythe output apparatus 40 ..'O ther modes of operation include theaccumulation of weights as described in detail below. I

In view of the above preliminary description of a portion of the systemas presented in FIG. 2, the operation of the system may new best beunderstood by explaining exemplary operations concurrently with theintroduction of other elements. Therefor, assume initially that the load10 (FIG. 1) along with the weight of the container 11 produces a weightindicative signal that is developed by the bridge circuit 43 (FIG. 2)and applied through the amplifier 86 and the filter 88 tothedifferential comparator 90 and thethreshold circuit 45. it is to beunderstood that this weightindicating signal manifests the gross weightof the load 10 and the container 11.

It is to be noted that the threshold circuit 45 is connected to theconductors 52 and 54 fromthe power supply 42. The

signal potential and the power supply. potential, (as received by thethreshold circuit) are ratioed,--as well known in the prior art, todevelop a signal that is truly representative of the gross weight andwhich, on exceeding apredeterrnined level, actuates the signal lamp 29to indicate an overload.

Assuming initially that net weightsareito be indicated, the unit is setso that the dial 27 (FIG; 1) indicates the known tare weight of thecontainer 11. As indicated, the dial 27 may be replaced bya card readeror thelike for installations in which the container is a mobile unitidentified by a card. However, in

' any event, the dial 27 (or an equivalent structure) controls a tareweight register 99 (FIG. 2) which provides digital signalsrepresentative of the tare weightofthe container 11. Structurally, thegenerator 99 may comprise simply a binary-codedmulticontact decimalswitch structure, controlled in association with the dial 27 as wellknown inthe prior art to provide representative digital signals. 1 l

The output from the register 99is-supplied through a cable 101 and anAND gate 102 to an arithmetic unit 103 which subtracts the valuerepresented by such sig'n'als from the value represented by gross-weightsignals received from the register 97 through a cable 105 and an ANDgate 106. The arithmetic unit may comprise a subtraction unit inassociation with an accumulator, various forms of which are well knownin the art, as disclosed in a book entitled Arithmetic Operations inDigital Computers by R.K. Richards, published in 1055, by D. VanNostrand Company, Inc.

The arithmetic unit 103 may be programmed to accumulate sets of digitalsignals for display therefrom by the output apparatus, or alternatively,may clear to hold only the lastreceived set of digital signals. Thetransfer of signals to the arithmetic unit 103 from the registers 97 and99 is controlled by a transfer signal, the development of which isconsidered below. The transfer from the tare register 99 through thegate 102 is also conditioned by closure of the net/gross switch (FlGS. 1and 2) WHICH which the gate 102 when net values are to be achieved.

' ln operation,.the arithmetic unit is commanded to accumulate eachweight (by adding it to the existing value in the unit) when theaccumulate/single read" switch 32 (FIGS 1 and 2) is closed. On thecontrary, if the switch 32 remains open, a

7 lower signal level is applied to the accur'nulatc" input of thearithmetic unit with the result that each weight observed is registeredindependently, thereby clearing the prior contents of the arithmeticunit.

. Assume initially, for example, that it is desired to read individual(noncumulatively) net weights. Each cycle is initiated by closure of theswitch 110 (FIG. 2) which supplies a highlevel signal to set a flip-flop112.'Whenth'e flip-flop l12'is set, an output therefrom qualifies an ANDgate 114 which is also connected to receive pulses from acloc'kgenerator 116. As a result, during the-interval when the flip-flop 112is set, clock pulses are applied to the gross register 97. lt is to benoted that 'thegross register is cleared preparatory to a cycle ofoperation by a transfer signal, the development of which is explainedhereinafter.

During the interval when the gross .register 97 is commended to step,the ramp generator (started, by the signal from the. switch 110)provides a substantially uniformly increasing potential through aconductor 121 to the differential comparator for comparison with theweight-indicating signal potential received through the conductors 92.The differential comparator 90 provides a low'output toa conductor 122as long a the ramp signal (carried in conductors 121) is less than theload signal carried-in the, conductors 92. However, as well known in theprior art'forcir'cuits of this type, immediately upon the ramp signalattaining the amplitude of the load signal, the differential comparator90 provides a high output through a conductor 122 to the flip-flop 112thereby resetting the flip-flop and inturn causing the output to thegate 114, to become low and thereby inhibiting the gate 114 ceasing theapplication of stepping pulses to-the gross register 97.

In that manner, the register 97 is driven by regularlyspaced clockpulses during an interval whichis dependent upon the amplitude of theload signal. Asa result, the gross-load analogue signal is converted toa time'base'which is in turn dissected into a digital count which istallied bythe register 97.

The gross weight digital signals from the register 97 are supplied tothe arithmetic unit 103 along with the tare digital signals from theregister 99 uponthe qualification of the gates 106 and 102 by thetransfer signal. That signal, is the signal manifesting amplitudecoincidence from the comparator 90, somewhat delayed by a delay circuit.124. That is a delayed signal from the circuit 124 is supplied to thegates 102 and 106 to command the transfer of number-representing signalsto the arithmetic unit 103. If, as assumed, 'net weight is desired, thetare (as represented by the signals passing through'the gate 102) issubtracted from the gross .(as' represented by the signals passingthrough the gate l06). Tl-le resulting signals (representing net) areheld in a registerwithin the arithmetic unit 103 which includes a signbit as well known in the art. The output apparatus 40 is connected tothat register to provide the desired output in a usable form. I

lf, in the operation of the system it is desired to accumulate aplurality of individually-observed weights, the switch 32 is closed. Asa result, an accumulate signal is'applied to the arithmetic unit '103commanding such a mode of operation. Consequently, the output registerof the arithmetic unit 103 is not cleared by incoming data; rather, suchincoming date is' added to the previously-held date-and the sum isretained for display or other manifestation by the output apparatus 40.As indicated, the output apparatus exhibits a numerical value as storedin the arithmetic unit 103 and received through a cable 105.Additionally, a sign bit which is registered in the unit 103 is alsosupplied to the output apparatus (through conductor 107) to be manifest.I 7

in the operation of the system to accumulate individually sensedweights, the possibility exist that the capacity of the output apparatuswill be exceeded. To provide an indication of such an occurrence, anoverflow output from the output register in the arithmetic unit 103 isconnected to the overflow signal lamp 29 through an AND gate 111, whichis qualified by the accumulate signal from the switch 32. Thus, shouldthe arithmetic unit 103 overflow during-the accumulate" mode ofoperation, the signal lamp 29 will be energized to indicate an excess.

Considering now certain detailed aspects of the system, it is to benoted that in the development of the gross-weight digital signals, asthe ramp generator 60 is driven by the power supply 42 which also drivesthe bridge 43, variations in the power supply tend to be compensated.That is, a slight drop in the reference potential from the power supply42 slightly reduces the amplitude of the weight-indicating load signalfrom the bridge 43 and concurrently reduces the slope of the output fromthe ramp generator 60 in a compensatory fashion.

The system, as disclosed also incorporates structure to perform acalibration which may be periodically verified to detect any changes orvariations that may affect the accuracy of the system. The systemutilizes the signal from the potentiometer 81 as a test signal toprovide a representative digital value in the counter 97 to verify priorcalibration. More specifically, in calibrating the system, the tap 82.isadjusted on the potentiometer 81 to provide a desired test signal.During such a calibration operation, the movable switch contacts 74 and76 are set in the lower position so that the potential across a dividedportion of the potentiometer 81 isapplied through the amplifier 86 andthe filter 88 to the differential comparator 90' in the same manner asan actual load signal. The test signal is then converted to provide adigital manifestation from the register 97 which is exhibited by theoutputapparatus 40. Normally, the potentiometer tap 82 will be adjustedso that a fullscale reading is provided. in subsequent'use of thesystem, this test may be repeated very simply and easily to verify theoperating stability and calibration.

As indicated above, various components of the system may comprisecircuits and subsystems as well-known in the prior art. However, aparticular form of the filter 88 is disclosed indetail and has beenfound to operate quite effectively. Specifically, the filter 88 is alow-pass active filter for eliminating noise components form the loadsignal; In this regard, one output line from the amplifier 86 isconnected through a resistor 120 to one of the lines 92. The resistor120 is connected through a capacitor 122 to a frequency-responsiveamplifier 124, the output of which is connected through anothercapacitor 126 back to the input, and through a resistor 128 to ajunction point 130. The junction between the input to the amplifier 124and the capacitor 122 is then connected through a resistor 134 to thejunction point 130. Lastly, the junction point 130 is connected througha capacitor 139 back to the reference output from the amplifier 86.Thus, a three-pole active filter is provided. In this regard, it is tobe noted that five pole filters have been effectively employed in thesystem. However, it is critically significant to employ a multipole,active filter.

As another specific element of the system of FIG. 2, the gross register97 has been described simply as a counter, e.g binary-coded decimalcounter. However, as suggested above, that unit includes means todigitally scale, i.e. multiply the output by values of 2 or 5.structurally, the counter or register 97 may be as shown in FIG. 3.Specifically, input pulses are supplied through a scaling switch 131having a movable contact 132 to individually engage any of itsstationary contacts 133, 134 or 135. The input pulses may thus bedirected to various binary scalar stages to accomplish scaling.

The contact 133 is connected to a binary, or scale-of-two'- unit 137which provides one output pulse to a conductor 138 for every tworeceived pulses. The conductor 138 isconnected to a scale-of-five unit139 (as well known in the art) for providing an output pulse toconductor 140, for every five input pulses received. The input conductor138 is also connected directly to the stationary contact 134, while theoutput conductor 140 is connected to a scale-of-ten unit 142 andreceives pulses form the contact 135 through a binary 144. The unit 142is connected to a series of similar units, e.g. unit 148 interconnecteda conventional binary-coded decimal stages.

In the operation of the register as shown in FIG. 3, when the movablecontact 132 engages the stationary contact 133, input pulses are scaledby 10 by the units 137 and 139. Consequently, these units functionjointly as a single binarycoded decimal unit with output terminals 150as indicated and overflow to the next digital stage, unit 142.

When the movable contact 132 is engaged with the contact 134, the binary137 is eliminated with the result that pulse trains (and resultantdecimal values) are multiplied by 2. A multiplication by 5 may beaccomplished by setting the movable contact 132 on the stationarycontact 135, in which position pulses are shunted around units 137 and139 through the binary 144. in that manner, the least-significant digitstage of the register is employed.- to accomplish digital scaling andgreater flexibility of operation.

In operation, basically an RC filter is provided, with the additionalconsideration that the capacitance therein varies with frequency. As aresult, the low-pass filter operating upon the input has a considerablysharpercutoff for output signals appearing from the amplifier 86. Thegeneral explanation of this improvement resides in accomplishing a,variable capacitance with changes in frequency, which is in turn relatedto accomplishing a variable gain in the amplifier 124 with changes infrequency as well-known in the prior art. These two variable factors areinterrelated as a result of mutual cooperation between the amplifier 124and the capacitor 122. As a result, alternating components of thesignal, i.e. noise from the amplifier 86 are shunted.

Summarizing the system is an effective weighting apparatus that may beemployed to accomplish accurate digital signals, indicative of netloads, gross loads, or accumulations and combinations of each. Thedigital representations are ratioed and may be sealed. The system tendsto be somewhat self-correcting with regard to variations in thereference potential from the power supply and additionally enablesfrequent and convenient calibration checks to verify current operation.

I claim: 1. A system for weighing an applied load, comprising: aplatform for receiving said load; a source of reference potential;

force transducer means for supporting said platform, said forcetransducer means having varying electrical characteristics in accordancewith the load supported thereby;

means for energizing said force transducer means with said referencepotential, whereby to develop an electrical load signal;

a ramp generator energized by said source of reference potential;

a digital counter for counting on a time. base; and

means for comparing said electricalv load signal with the output of saidramp generator whereby to establish an operating level for said digitalcounter.

2. A system according to claim 1 further including:

means for providing digital signals representative of a tare weightportion of said load; and

means for arithmetically combining signals indicative of the content ofsaid digital counter and said digital signals representative of a tareweight.

3. A system according to claim 2 wherein said means for combiningincludes an arithmetic unit connected to receive said digital signalsrepresentative of a tare weight and also connected to said counter.

4. A system according to claim 3 further including means for applying aportion of said reference potential to said digital means whereby totest the operation of said system.

5. A system according to claim 4 further including multipole activefilter means for filtering said electrical load signal prior toapplication thereof to said digital means. i

6. A system according to claim 1, further including means for applying aportion of said reference potential to said digital means whereby totest the calibration of said system.

7. A system according to claim 1 further including means to accumulatesaid signals indicative of the content of said digital counter.

3 ,5 65 1 94 7 8 v 8. A system according to claim I including means toscale counter and further including means for applying a portion of theoperation of said digital counter. said reference potential to saiddigital means whereby to test 9. A system according to claim 8, furtherincluding means to the operation of said system. i

accumulate said signals indicative of the content of said digital

1. A system for weighing an applied load, comprising: a platform forreceiving said load; a source of reference potential; force transducermeans for supporting said platform, said force transducer means havingvarying electrical characteristics in accordance with the load supportedthereby; means for energizing said force transducer means with saidreference potential, whereby to develop an electrical load signal; aramp generator energized by said source of reference potential; adigital counter for counting on a time base; and means for comparingsaid electrical load signal with the output of said ramp generatorwhereby to establish an operating level for said digital counter.
 2. Asystem according to claim 1 further including: means for providingdigital signals representative of a tare weight portion of said load;and means for arithmetically combining signals indicative of the contentof said digital counter and said digital signals representative of atare weight.
 3. A system according to claim 2 wherein said means forcombining includes an arithmetic unit connected to receive said digitalsignals representative of a tare weight and also connected to saidcounter.
 4. A system according to claim 3 further including means forapplying a portion of said reference potential to said digital meanswhereby to test the operation of said system.
 5. A system according toclaim 4 further including multipole active filter means for filteringsaid electrical load signal prior to application thereof to said digitalmeans.
 6. A system according to claim 1, further including means forapplying a portion of said reference potential to said digital meanswhereby to test the calibration of said system.
 7. A system according toclaim 1 further including means to accumulate said signals indicative ofthe content of said digital counter.
 8. A system according to claim lincluding means to scale the operation of said digital counter.
 9. Asystem according to claim 8, further including means to accumulate saidsignals indicative of the content of said digital counter and furtherincluding means for applying a portion of said reference potential tosaid digital means whereby to test the operation of said system.